1. Field of the Invention
The present invention relates to an electrophotographic printer.
2. Description of Related Art
With an electrophotographic printer, a charging device supplies static charges to a photosensitive drum. An LED head illuminates the photosensitive drum to form an electrostatic latent image on the photosensitive drum. Then, the electrostatic latent image is developed into a toner image by a developer. The toner image is then transferred to a print medium such as paper by means of a transferring device.
FIG. 9 illustrates a general construction of a prior art electrophotographic printer.
The photosensitive drum 11 rotates in a direction shown by arrow A. When the photosensitive drum 11 rotates, the charging roller 12, developing roller 15, transfer roller 18, and cleaning roller 13 are rotated in directions shown by arrows B, C, D, and E, respectively.
FIG. 10 illustrates a sequence of voltages on the respective rollers 12 and 15 when the printer shown in FIG. 9 performs printing operation. The shaft 12a of the charging roller 12 receives a charging voltage V.sub.A from a power supply 20 and the shaft 15a of the developing roller 15 receives a bias voltage V.sub.B from a power supply 21. The sequence of the voltages applied to the shafts 12a and 15a will be described.
Referring to FIG. 10, the charging voltage V.sub.C is a voltage on a surface area of the photosensitive drum 11 in contact with the developing roller 15.
Upon a print command, the power supply 20 supplies a negative charging voltage V.sub.A =-V.sub.0 to the shaft 12a. At this time, the surface region H (FIG. 9) of the photosensitive drum 11 downstream of the charging roller 12 but upstream of the developing roller 15 has not been charged yet.
During the time period t from the application of the negative charging voltage V.sub.A =-V.sub.0 till the trailing end of the surface region H (FIG. 9) passes the developing roller 15, the power supply 21 supplies a positive bias voltage V.sub.B =+V.sub.1 to the shaft 15a. This positive bias voltage +V.sub.1 prevents toner particles from adhering to the photosensitive drum 11. After the time period t, the surface area of the photosensitive drum 11 which is brought into contact with the developing roller 15 has been negatively charged by the charging roller 12. The power supply 21 supplies a negative bias voltage V.sub.B =-V.sub.2 to the shaft 15a so that the toner particles are deposited on the photosensitive drum 11.
When performing continuous printing operation, the photosensitive drum 11 is rotated continuously and the power supply 21 continues to supply the negative bias voltage V.sub.B to the shaft 15a.
However, if one printing cycle is followed by the next printing cycle with a relatively short time interval therebetween, the following printing operation starts before the potential V.sub.C of the surface area of the photosensitive drum 11 in contact with the developing roller 15 has been discharged to nearly zero volts (shown by dotted line in FIG. 10). On the other hand, the bias voltage V.sub.B is set to +V.sub.1 upon a print command, so that the potential difference between the photosensitive drum 11 and the developing roller 15 increases to a value greater than a firing potential.
Therefore, discharge takes place across the photosensitive drum 11 and developing roller 15 with the result that toner particles are reversely charged and clump by Coulomb force. This reduces the amount of toner which adheres to the developing roller 15, resulting in a decrease in print density.